How exactly does a UPS emergency power system work and why?

How exactly does a UPS emergency power system work and why…do you need one?

Most of our clients who are computer professionals or otherwise work in datacenters are savvy as to why they need a backup power supply for servers. However, if you come to IT from the software side—or if you’re not an electrical engineer or technician—what exactly your UPS power conditioner with battery backup does and why it’s needed might surprise you. Conversely, if you’re expert in the how & why of your UPS battery backup system, then maybe you can pass this article on to one of your stakeholders who needs and wants to know.

As to the why…

The AC current produced by utility companies in America is 60 Hz (cycles per second), meaning that it changes polarity 60 times per second. Doing so (in the case of 120 VAC), the voltage actually goes up and down from +120 to -120 volts in that split second, a range of 240 volts. It’s the same for other voltages you might have in your computer datacenter, whether single-phase or three-phase.

If you were to apply an oscilloscope to that current, you would see that these up & down excursions form a classic sinusoidal wave. But because power grid technology was developed mostly in the early 20th Century, it is not particularly friendly to 21st Century computers or mission-critical equipment. There are instances when line voltage is greater than it should be, called surges and spikes; and there are times when it’s less than it should be, called sags.

Surges, spikes, and sags will distort the sinusoidal wave you see on the oscilloscope. (This is why technicians and engineers speak of “sinusoidal purity” when discussing the slings & arrows of your AC power supply.) A toaster, hairdryer, vacuum cleaner, or electric stove is oblivious to these distortions. But commercial and industrial computers, highly-sophisticated manufacturing apparatus, and a raft of medical equipment are not so tolerant. They shut down in the face of voltage variations, which can be inconvenient at best and fatal at worse.

As to the how…

There are three kinds of industrial UPS systems or commercial UPS systems.

  • Offline-Standby UPS is the simplest. It’s essentially a battery backup, and it only provides surge & spike protection against complete power failures and voltage spikes and dips
  • Line-Interactive UPS is similar to Office-Standby, but it uses a specialized multi-tap transformer that reacts to voltage changes by increasing or reducing its magnetic field and, hence, the power output into the load circuit. This gives it two advantages over the Office-Standby. It protects against voltage sags and dips as well as against voltage surges and spikes; and it does so using line current instead of its battery pack—in the short run increasing the available runtime of the UPS during a total power failure, and in the long run, extending the useful life of the battery pack.
  • Online Double-Conversion UPS is the most sophisticated. To understand how it works, it helps to be familiar with two concepts:
    1. AC Voltage Rectification—batteries cannot be recharged with AC current and so all UPS battery backup systems include a rectifier that changes the AC to DC.
    2. DC Voltage Inversion—however, when the battery pack is called upon during sags and dips or a total power failure, it can only deliver DC. So an inverter is required to transform the DC to AC

In this sort of UPS battery backup system, the battery pack is always connected to the inverter, so that no power transfer switches are necessary that might contaminate the “sinusoidal purity” of the AC current. During a complete power outage, the rectifier simply drops out of the circuit, leaving the battery pack to keep the power steady and unchanged. When power is restored, the rectifier resumes carrying most of the load while recharging the battery pack.

So what’s the upshot?

Because it’s best at maintaining the “sinusoidal purity” of the AC current, an online double-conversion uninterruptible power system is best for applications where electrical isolation is necessary, and/or for equipment that is very sensitive to voltage sags and dips and voltage surges and spikes.

In effect, it provides an “electrical firewall” between the incoming utility power and your sensitive electronic equipment.

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